As the trial got under way at The Hague, the chief prosecutor said the outcome would define a commander's legal responsibility to control his troops wherever they are in the world. Campaigners also hope it will send warning that those who use rape as a weapon of war will be caught and punished.

Just four years ago Bemba, a protege of former dictator Mobutu Sese Seko, was a multimillionaire businessman and darling of the political opposition in the Democratic Republic of Congo.

But following his arrest in Belgium in 2008, the 48-year-old today looked sombre as he sat at only the third ICC trial since it began work in 2002.

Bemba, who was vice-president of Congo from 2003 to 2006, is the most senior political figure in the court's custody. He was defeated in the country's last presidential election, but had been seen as likely to run again next year. His supporters in Congo claim the case against him is politically motivated.

Prosecutors say Bemba allowed his personal militia, the Movement for the Liberation of Congo, to run amok in neighbouring Central African Republic in 2002 and 2003 after its then-president, Ange-Félix Patassé, asked for its help in an ultimately unsuccessful fight against rebels led by Congo's former army chief of staff François Bozizé.

Bemba pleaded not guilty to three counts of war crimes and two of crimes against humanity covering murder, rape and pillaging by his forces.

Prosecutor Luis Moreno-Ocampo said the case will set a precedent for the conduct of war everywhere. "The judges' definition of the responsibilities of a commander will be a warning for all the military commanders in the world," he said before the hearing. "They have to understand what is legal and what is illegal. The law makes a difference between a commander and a criminal."

The prosecutor added that the trial would shine a spotlight on "neglected [victims] in a neglected country" - girls, women and men raped in Central African Republic - and described it as a "crime of domination and humiliation".

His deputy, Fatou Bensouda, told judges at a preliminary hearing: "[Bemba] chose rape as his method." She cited one unnamed witness who said three of Bemba's men burst into his home firing weapons, raped him for hours in front of his family, then raped his wife and attacked his children. The witness said: "In front of my eyes they abused my wife. After they finished with my wife, they came for my kids."

Prosecutors plan to call up to 40 witnesses, including more than a dozen rape victims, and expect to take six months to present their evidence.

Human Rights Watch welcomed the trial. Anneke Van Woudenberg, a senior researcher in its Africa division, said: "It's significant in that he is the most high-profile individual the ICC is dealing with. In DR Congo this trial is going to be watched very closely. Everyone there knows who he is."

She added: "Every one of these ICC trials is crucial because they show that no one is above the law and justice is being done. This is important in particular for the people of the Central African Republic, although we have been urging that the charges be extended to DR Congo, where horrific atrocities were also committed."

Brigid Inder, executive director of Women's Initiatives for Gender Justice, said the trial should show women "that crimes of sexual violence are important enough to prosecute those who commit such acts" and demonstrate to military and militia leaders "that should they fail to prevent or punish subordinates for gender-based crimes, they will be held accountable".

Bemba's lawyers say he has tried to investigate and prosecute officers responsible for the atrocities. Moreno-Ocampo dismissed those efforts as "sham trials".

One of Bemba's lawyers, Aime Kilolo, said his client was "very serene" on the eve of the trial. "He will show that at no time could he be associated with allowing rapes or murders," Kilolo said. "Nor was he associated with men in the field, nor did he have effective control of troops, nor was he responsible for discipline."

Bemba's lead lawyer, Nkwebe Liriss, said the court had frozen his client's assets and not provided his defence team with enough money to carry out any investigations.

Bemba ruled a large part of Congo during that country's 1998-2002 war with support from neighbouring Uganda. After a peace agreement ended the war, he became one of Congo's four vice-presidents.

He came second in a presidential election in 2006 behind Joseph Kabila . He was elected a senator but refused to dismantle his militia, which led to clashes with security forces that left at least 300 dead in March 2007. Facing charges of treason, he fled into exile in Portugal and Belgium.

Bemba was arrested in Brussels and transferred to the court in The Hague in July 2008. The two previous ICC cases to go to trial concern the conflict in Congo. The former Liberian president Charles Taylor is being tried at the The Hague but in a special war crimes tribunal for Sierra Leone rather than the ICC.